1. Field of the Invention
The invention relates to a frequency synthesizer for generating a low noise and low jitter time base of a reference signal with the frequency fref, wherein the time base consists of a first output signal with the frequency f1 and a second output signal with the frequency f2, having a first fractional divider with the division factor T1 for generating the first output signal from the reference signal, a second fractional divider with the division factor T2 for generating the second output signal from the reference signal and a controller for periodically-clocked switching of the first fractional divider between T1 and (T1+1) using the first output signal and for periodically-clocked switching of the second fractional divider between T2 and (T2+1) using the second output signal.
2. Description of Related Art
Such frequency synthesizers are particularly suitable for providing the time base for a fill level measuring device, which determines the fill level of a medium in a container by means of time domain reflectometry (TDR). Subsequently, the frequency synthesizer of interest is illustrated by this example of use, of course, it is not limited to such use. Fill level measuring devices operating with time domain reflectometry have at least one conductor extending generally from above into the container and the medium, whose fill level is to be determined. The fill level measuring device emits electromagnetic signals along the conductor in the direction of the medium and receives the components of the signals that have been reflected back from the interface layer of the medium, i.e., on the surface of the medium, to the level measuring device. The medium can be e.g., liquids, powders or granules. The reflected proportion of the signal depends on the dielectric constant of the medium and increases with this. The running time of a signal, which is the length of time that the signal requires from the fill level measuring device to the surface of the medium and the reflected portion of the signal back to the fill level measuring device, is proportional to the distance of the fill level measuring device to the surface of the medium. The fill level is calculated using the distance of the medium from the fill level measuring device, together with the geometry of the container. Changing environmental conditions, such as a rising or falling ambient pressure or a rising or falling temperature, do not affect measurement accuracy in fill level measuring devices using time domain reflectometry. Also, the transit time of the signals is independent of the dielectric constant of the medium on the surface of which the reflection takes place. Time domain reflectometry is also e.g., used in cable testing and its principle is similar to radar methods.
Time domain reflectometry is accordingly based on the measurement of the transit time of an electromagnetic signal. If the container is nearly completely filled with medium, so that the surface of the medium is, for example, only 15 cm below the fill level measuring device, then the total transit distance of the electromagnetic signal from the fill level meter to the surface of the medium and back to the fill level measuring device is only 30 cm, which corresponds to a duration of the electromagnetic signal of 10 ns. The measurement of such short time spans for determining the fill level is associated with high effort and associated costs, which is why scanning of the electromagnetic signals required for time measurement is carried out by sub-sampling. For the sub-scan, a time base is required with two high-frequency signals, whose frequency difference is as low as possible.
The aforementioned frequency synthesizer provides this time base with the first output signal having the frequency f1 and the second output signal having the frequency f2, wherein the difference frequency is Δf=f1−f2. Without limiting the generality of illustration, it is assumed here that the frequency f1 is larger than the frequency f2. Individual electromagnetic signals are emitted at a transmission rate in the direction of the medium, which corresponds to the frequency f1 and the electromagnetic signals, from which the transit time is calculated and which belong to waves reflected at the surface of the medium, are sampled at a sampling rate that corresponds to the frequency f2. The frequencies f1 and f2 are typically in the range of a few megahertz, and the difference frequency is usually in the range of a few 100 Hz. Due to the high transmission rate and the resulting low temporal spacing of individual, successive, emitted electromagnetic signals, reflected electromagnetic signals successively received from the fill level measuring device are virtually identical in the sense that the reflected signals are not affected by changes in the fill level, because changes in the fill level take place very slowly in relation to the transmission rate. Since the scanning of the electromagnetic signals is carried out with a sampling rate that is only slightly below the transmission rate, the sampling of successive signals is shifted slightly, so that sub-scanning in conjunction with virtually unchanged reflected signals results in an accurate picture of the signals, which is used to calculate the transit time. The transit time of the signals, and in turn the fill level can be calculated using the sub-sampled signals. A time base that is low-noise and low-jitter is indispensable for high accuracy in sub-sampling. Noise would degrade the signal-to-noise ratio and jitter would lead to uncertainty in the sampling.
German Patent Application DE 102 44 348 A1 describes a frequency synthesizer with a reference oscillator that generates a first output signal having the frequency f1 and supplies the first output signal frequency-divided by the division factor M to a mixer. A control oscillator produces a second output signal having the frequency f2 and the second output signal frequency-divided by the division factor N is also supplied to the mixer. The signal mixed from the frequency-divided first output signal and the frequency-divided second output signal is supplied to a phase-frequency discriminator, which evaluates this signal and the first output signal frequency-divided by the division factor L. The controlled variable of the phase frequency discriminator is filtered by a loop filter and is the control input variable of the control oscillator. In practice, it has been shown that the phase-locked loop has a relatively high momentum, but the achievable stability and the associated noise behavior needs improvement. In addition, the cost in terms of circuit technology is considerable.
A frequency synthesizer is known from German Patent Application DE 10 2004 063 935 A1 comprising a reference oscillator and a first frequency divider with the division factor V1 connected downstream from the reference oscillator for output of the first output signal with the frequency f1 and comprising a control oscillator and a second frequency divider with the division factor of V2 connected downstream from the control oscillator for output of the second output signal having the frequency f2. The output signal of the reference oscillator is divided by a third frequency divider with the division factor M and supplied to a mixer. Accordingly, the output signal of the control oscillator is divided by a fourth frequency divider with the division factor N and supplied to the mixer. The output signal of the mixer is one of two input signals of a phase-frequency discriminator. The other input signal of the phase frequency discriminator is the output signal of the reference oscillator divided by a fifth frequency divider with the division factor L. The controlled variable of the phase frequency discriminator is fed to a loop filter and the output signal of the loop filter is the controlled variable for the control oscillator. The output of the first output signal by the first frequency divider and the second output signal by means of the second frequency divider allows that the two oscillators, namely the reference oscillator and the control oscillator, disturb each other much less due to coupling effects since the two oscillators operate now with widely varying frequencies. However, high demands are placed on the mixer in respect to noise characteristics, thus resulting in considerable analog circuitry.
Furthermore, a frequency synthesizer is known from German Patent Application DE 10 2010 011 128 A1 comprising a reference oscillator and an integral first frequency divider with dividing factor V1 connected downstream from the reference oscillator for producing the first output signal with the frequency f1 and a control oscillator with an integral second frequency divider with the division factor of V2 connected downstream from the control oscillator for generating the second output signal having the frequency f2. The signal of the reference oscillator divided by a fractional third frequency divider with the division factor R is supplied to a phase-frequency discriminator. Further, the phase-frequency discriminator is also supplied with the signal of the reference oscillator divided by a fractional frequency divider with the third division factor L. The controlled variable of the phase frequency discriminator is filtered by a loop filter and controls the control oscillator.
Although it is possible to generate very small difference frequencies with this frequency synthesizer, this involves considerable complexity and also goes hand in hand with corresponding costs. A phase-locked loop comprising the phase-frequency discriminator, the loop filter, the control oscillator, and the third frequency divider is also required for the function of the frequency synthesizer.